Method and apparatus for configuring search space of a downlink control channel

ABSTRACT

A method for configuring a search space of a downlink control channel is provided. The method includes determining the parameters for Enhanced Physical Downlink Control Channel (E-PDCCH) candidates of each aggregate level according to the number of Resource Elements (RE) in a subframe and/or the number of bits of Downlink Control Information (DCI) formats, when the parameters for E-PDCCH candidates of E-PDCCH search space is configured, determining, by a User Equipment (UE), the parameters for E-PDCCH candidates according to a current downlink subframe and a detected DCI format, and detecting blindly, by the UE, the E-PDCCH candidates in the E-PDCCH search space corresponding to the parameters for E-PDCCH candidates. The present invention also provides a UE and a base station. Application of the present invention can improve the flexibility of the base station scheduling, and reduce the possibility that the E-PDCCHs of different UEs block each other.

PRIORITY

This application claims the benefit under 35 U.S.C. §119(a) of a Chinesepatent application filed on Sep. 4, 2012 in the Chinese IntellectualProperty Office and assigned Serial No. 201210323726.X and of a Chinesepatent application filed on Mar. 19, 2012 in the Chinese IntellectualProperty Office and assigned Serial No. 201210076438.9, the entiredisclosure of each of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communication systemtechnology. More particularly, the present invention relates to a methodand apparatus for configuring a search space of a physical downlinkcontrol channel.

2. Description of the Related Art

In a wireless communication system, downlink transmission refers to abase station's transmission of a signal to a User Equipment (UE). Adownlink transmission signal includes a data signal, a control signaland a reference signal (i.e., Pilot). Herein, the base station transmitsdownlink data in a Physical Downlink Shared Channel (PDSCH), ortransmits downlink control information in a downlink control channel.Uplink transmission refers to transmission of a signal by the UE to thebase station. An uplink signal also includes a data signal, a controlsignal and a reference signal. Herein, the UE transmits uplink data in aPhysical Uplink Shared Channel (PUSCH), or transmits uplink controlinformation in a Physical Uplink Control Channel (PUCCH). The basestation may dynamically schedule PDSCH transmission and PUSCHtransmission of the UE by a Physical Downlink Control Channel (PDCCH).In a 3GPP LTE system, downlink transmission is performed usingOrthogonal Frequency Division Multiple Access (OFDMA) technology, anduplink transmission is performed using Signal-Carrier Frequency DivisionMultiple Access (SCFDMA) technology. In the 3GPP LTE system, the lengthof each wireless frame, which is equally divided into 10 subframes, is10 ms and a downlink Transmission Time Interval (TTI) is defined as onesubframe

FIG. 1 is a schematic diagram illustrating a subframe structure in aLong Term Evolution (LTE) system according to the related art.

Referring to FIG. 1, each downlink subframe includes two time slots. Fora normal Cyclic Prefix (CP) length, each time slot contains 7 OFDMsymbols; for an extended CP length, each time slot contains 6 OFDMsymbols. In each subframe, the first n OFDM symbols, where n is equal to1, 2 or 3, are used for transmitting the downlink control informationincluding the PDCCH and other control information; the remaining OFDMsymbols are used for transmitting the PDSCH.

Resource allocation is based on a Physical Resource Block (PRB), whereinone PRB contains 12 consecutive sub-carriers in the frequency domain andcorresponds to one time slot in the time domain. Two PRBs in two timeslots of the same sub-carrier in one subframe constitute a PRB pair. Ineach PRB pair, a Resource Element (RE) is a minimal unit of time andfrequency resource. That is, the RE contains one sub-carrier in thefrequency domain and contains one OFDM symbol in the time domain. REsmay be respectively used for different functionalities. For example,some of the REs may be used respectively for transmitting a Cellspecific Reference Signal (CRS), a user specific Demodulation ReferenceSignal (DMRS), a Channel State Information Reference Signal (CSI-RS)etc.

In the LTE system, multiple data transmission modes are defined. Forexample, in the downlink direction, the data transmission modes includea closed loop Multiple Input Multiple Output (MIMO) transmission mode,an open loop MIMO transmission mode, a transmitting diversitytransmission mode etc. In the uplink direction, the transmission modesinclude a single antenna transmitting mode, a MIMO mode, etc. For onetransmission mode, the system configures a normal Downlink ControlInformation (DCI) format used for normal data transmission in thetransmission mode. At the same time, the base station configures the UEto check a fallback DCI format having a small number of bits and that isused for scheduling data in a conservative manner, e.g., transmittingdiversity or data transmission through a single antenna, thereby havinghigh reliability. In addition, DCI formats of the uplink transmissionand the downlink transmission are different. That is, in one TTI, the UEneeds to detect multiple possible DCI formats.

In the LTE system, DCI transmitted to different UEs or DCI of differentfunctionalities is coded independently and transmitted. When the PDCCHis mapped to physical resources, a Control Channel Element (CCE) istaken as a unit, i.e., one PDCCH modulation symbol may be mapped to LCCEs where L is equal to 1, 2, 4, or 8, and L is also called a PDCCHaggregate level. Each CCE contains 36 REs. Since the PDCCH usesQuad-Phase Shift Keying (QPSK) as a modulation method, the base stationmay select the aggregate level of the CCE for transmitting the PDCCHaccording to the number of bits of the control information and the UE'slink condition.

Herein, if each UE is configured with a unique PDCCH, when the quantityof UEs exceeds the quantity of PDCCHs, a problem is caused that thePDCCHs of the UEs block each other. Otherwise, if all of the PDCCHs maybe allowed to be configured to all of the UEs, the UEs need to beconfigured to detect all of the possible PDCCHs. As a consequence, theUE's complexity is increased and a false alarm rate is caused to beincreased. Thus, in the LTE system, the UE is configured to detect thePDCCH at multiple possible locations, which are called a UE searchspace. The base station transmits the PDCCH on one of the multiplepossible locations configured for UE detection. By blindly detecting themultiple locations configured by the base station, the UE may obtain thecontrol information transmitted by the base station on one of thelocations. In the LTE system, the UE needs to detect the PDCCHrespectively in a cell Common Search Space (CSS) and a UE specificSearch Space (USS). Herein, the PDCCH in the CCS is usually used forscheduling system broadcast information etc., and the PDCCH in the USSis usually used for dynamically scheduling the PDSCH and PUSCH of theUE.

According to the above description, the UE needs to detect the CSS andthe USS respectively, and in each search space, the UE needs to detectmultiple possible DCI formats. In the LTE system, the number of PDCCHcandidates corresponding to each aggregate level for each type of theDCI formats is defined, as shown in Table 1. Herein, for the same searchspace, the number of PDCCH candidates needing to be detected for variousDCI formats are the same.

TABLE 1 Search Space S^((L)) The number of the PDCCH Type AggregateLevel L candidates M^((L)) UE specific 1 6 2 6 4 2 8 2 Cell common 4 4 82

According to Table 1, when the aggregate level is 1, the number of theaggregate PDCCHs is 6. Thus, the UE needs to blindly detect the PDCCH on6 possible locations.

In an enhanced version of LTE, the PDCCH may become a system performancebottleneck since it must support multi-cell joint transmission or aheterogeneous network. In order to support greater capacity of thecontrol channel, and to support interference collaboration of controlchannels of multiple cells, an enhanced PDCCH (E-PDCCH) is proposed.

FIG. 2 is a schematic diagram illustrating an existing multiplexingmethod of an E-PDCCH according to the related art.

Referring to FIG. 2, the E-PDCCH is mapped to a data region of thesubframe and multiplexed with the PDSCH by using Frequency DivisionMultiplexing (FDM).

As shown in FIG. 2, it is assumed that the E-PDCCH is started from theOFDM symbol that is adjacent to the PDCCH (actually the E-PDCCH mayalternatively be fixedly started from one OFDM symbol configured by ahigh layer), and the E-PDCCH occupies a certain number of OFDM symbols.The base station may notify the UE of the PRB pair used for transmittingthe E-PDCCH by a higher layer signal which may be cell specific or maybe specially transmitted to each UE respectively, and for different UEs,the PRB pairs used for the E-PDCCH may be different.

According to a method for mapping E-PDCCH resources, the E-PDCCH may bedivided into a localized E-PDCCH and a distributed E-PDCCH. When theaccurate Channel Quality Indication (CQI) of different frequencysub-bands of the UE can be obtained, the base station may choose totransmit the E-PDCCH in a suitable PRB pair to obtain frequencyscheduling gain, i.e., the localized E-PDCCH. Accordingly, when theaccurate CQI of the UE is not obtained, the base station has to transmitthe E-PDCCH in multiple PRB pairs to obtain frequency distribution gain,i.e., the distributed E-PDCCH. The distributed E-PDCCH may also be usedin a condition that the E-PDCCH needs to be transmitted to multiple UEs.

The E-PDCCH is also composed of Enhanced-CCEs (E-CCEs), which correspondto the CCEs that constitute the PDCCH. The UE also needs to detect oneor multiple E-PDCCH candidates in a certain search space correspondingto the search space of the PDCCH.

In each PRB pair, the number of REs that may be practically used fortransmitting the E-PDCCH is variable and relies on multiple situationssuch as:

-   -   a. the number of OFDM symbols in a backward compatible control        area, i.e., the number of the OFDM symbols occupied by the        downlink control information;    -   b. the number of REs occupied by the CRS that is transmitted in        the PDSCH area for the normal subframe but is not transmitted in        the PDSCH area for a Multicast Broadcast Single Frequency        Network (MBSFN) subframe;    -   c. the number of REs used for the DMRS; and    -   d. whether the CSI-RS is transmitted, etc.

The variation of the available REs in the PRB pair causes variation inthe size of the E-CCE. The variation range of the number of REs of theE-CCE may be reduced by adjusting the number of the divided E-CCEsaccording to the number of available REs, but this cannot thoroughlyavoid the variation of the number of REs.

FIG. 3 is a schematic diagram illustrating methods for dividing thelocalized E-PDCCH into E-CCEs according to the related art.

Referring to FIG. 3, it is assumed that each PRB pair is divided into 4E-CCEs. In example 1, a backward compatible control channel occupies 3OFDM symbols, a normal subframe structure is used, 4 CRS ports areconfigured, and the CSI-RS is configured. Thus, only 84 REs may be usedfor the E-PDCCH and each E-CCE only has 21 REs, on average. In example2, the backward compatible control channel occupies 2 OFDM symbols, anMBSFN subframe structure is used, 4 CRS ports are configured, and noCSI-RS is configured. Thus, there may be 120 REs used for the E-PDCCH,and each E-CCE contains 30 REs, on average. It can be seen that thesizes of the E-CCE in the two examples are substantially different.Actually, according to the simple method for dividing the E-CCE bysubcarriers, even in the same example, the sizes of the E-CCEs are notcompletely equal.

Accordingly, there is a need for a method and apparatus for configuringa search space of a downlink control channel to increase the flexibilityof base station scheduling and reduce the possibility that the E-PDCCHsof different UEs block each other.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present invention.

SUMMARY OF THE INVENTION

Aspects of the present invention are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentinvention is to provide a method and apparatus for configuring a searchspace of a downlink control channel to increase flexibility of basestation scheduling and reduce the possibility that the E-PDCCHs ofdifferent UEs block each other.

In accordance with an aspect of the present invention, a method forconfiguring a search space of a downlink control channel is provided.The method includes determining parameters for Enhanced PhysicalDownlink Control Channel (E-PDCCH) candidates of each of aggregatelevels according to the number of Resource Elements (RE) in a subframeand/or the number of bits of Downlink Control Information (DCI) formatswhen configuring the parameters for E-PDCCH candidates of an E-PDCCHsearch space, determining, by n User Equipment (UE), the parameters forE-PDCCH candidates according to a current downlink subframe and adetected DCI format, and detecting blindly, by the UE, the E-PDCCHcandidates in the E-PDCCH search space corresponding to the parametersfor E-PDCCH candidates.

In an exemplary implementation, at least two sets of parameters forE-PDCCH candidates are configured respectively for the aggregate levels.

In an exemplary implementation, different parameters for E-PDCCHcandidates are configured for different subframes according to thenumber of REs available for the E-PDCCH in each of the subframes, andthe same parameters for E-PDCCH candidates are adopted for all of theDCI formats in the same subframe.

In an exemplary implementation, different parameters for E-PDCCHcandidates are configured for different DCI formats according to thenumber of bits of each DCI format and the same parameters for E-PDCCHcandidates are used for one DCI format in all of subframes.

In an exemplary implementation, different parameters for E-PDCCHcandidates are configured for different DCI formats in differentsubframes according to the number of REs which may be used for theE-PDCCH in each subframe and the number of bits of each DCI format.

In an exemplary implementation, the DCI formats are grouped and the sameparameters for E-PDCCH candidates are configured for each group of theDCI formats.

In an exemplary implementation, at least two sets of the parameters forE-PDCCH candidates corresponding to each aggregate level are configuredaccording to whether the E-PDCCH of the aggregate level may carry atleast one DCI format to be transmitted.

In an exemplary implementation, a single set of standard parameters forE-PDCCH candidates are configured, the workable aggregate levels aredetermined according to the number of REs which may be used for theE-PDCCH in the subframe, and the parameters for E-PDCCH candidates ofeach aggregate level are obtained by translating the standard parametersfor E-PDCCH candidates.

In an exemplary implementation, the workable aggregate levels aredetermined for each DCI format, and the parameters for E-PDCCHcandidates of each aggregate level are obtained, or the workableaggregate levels are determined uniformly for all DCI formats in eachsubframe, and the parameters for E-PDCCH candidates of each aggregatelevel are obtained.

In an exemplary implementation, a method of translating includes settingthe number of the E-PDCCH candidates for the aggregate level(s) lessthan the minimal workable aggregate level to 0, setting the number ofthe E-PDCCH candidates for the minimal workable aggregate level to thefirst item of the numbers of the standard E-PDCCH candidates, settingthe number of the E-PDCCH candidates of the second minimal workableaggregate level to the second item of the numbers of the standardE-PDCCH candidates, and so on.

In an exemplary implementation, a variation of the number of REsavailable for the E-PDCCH in a PRB pair influences only the number ofthe E-PDCCH candidates in a localized E-PDCCH search space, or avariation of the number of REs available for the E-PDCCH in a PRB pairinfluences the number of the E-PDCCH candidates both in a localizedE-PDCCH search space and that in a distributed E-PDCCH search space.

In accordance with another aspect of the present invention, a userequipment is provided. The user equipment includes a parameter selectionmodule and a blind detection module, wherein the parameter selectionmodule is adapted to determine the parameters for E-PDCCH candidatesaccording to a current downlink subframe and a detected DCI format, andthe blind detection module is adapted to blindly detect an E-PDCCH in anE-PDCCH search space corresponding to the parameters for E-PDCCHcandidates determined by the parameter selection module.

In accordance with another aspect of the present invention, a basestation is provided. The base station includes a parameter selectionmodule and a sending module, wherein the parameter selection module isadapted to determine the parameters for E-PDCCH candidates according toa current downlink subframe and a detected DCI format, and the sendingmodule is adapted to select an E-PDCCH candidate in an E-PDCCH searchspace corresponding to the parameters for E-PDCCH candidates selected bythe parameter selection module, and to send downlink controlinformation.

It can be seen from the above technical solution for configuring thesearch space for the downlink control channel provided by exemplaryembodiments of the present invention, when the parameters for E-PDCCHcandidates of the E-PDCCH search space are configured, the parametersfor E-PDCCH candidates of each aggregate level are determined by thenumber of REs which may be used for the E-PDCCH in each subframe and/orthe number of bits of each DCI format, thus, different parameters forE-PDCCH candidates of E-PDCCH search space are used for differentsubframes and/or different DCI formats. In the subframes in which thenumber of REs that may be used for the E-PDCCH is small, the E-PDCCHs ofsome of the aggregate levels are unusable. However, by assigning theunusable E-PDCCH candidates to the other aggregate levels, the totalnumber of the E-PDCCH candidates is guaranteed not to be changed or tobe changed little. Thus, the flexibility with which the base stationtransmits the E-PDCCH is increased and the possibility that the E-PDCCHsof different UEs block each other is reduced.

Other aspects, advantages, and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainexemplary embodiments of the present invention will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a subframe structure in aLong Term Evolution (LTE) system according to the related art;

FIG. 2 is a schematic diagram illustrating an existing multiplexingmethod of an E-PDCCH according to the related art;

FIG. 3 is a schematic diagram illustrating distribution of ResourceElements (REs) used for an methods for dividing the localizedEnhanced-Physical Downlink Control Channel (E-PDCCH) into E-CCEsaccording to the related art;

FIG. 4 is a flowchart illustrating a method for configuring a searchspace for a downlink control channel according to an exemplaryembodiment of the present invention;

FIG. 5 is a schematic diagram illustrating a structure of a userequipment according to an exemplary embodiment of the present invention;

FIG. 6 is a schematic diagram illustrating a structure of a base stationaccording to an exemplary embodiment of the present invention;

Throughout the drawings, it should be noted that like reference numbersare used to depict the same or similar elements, features, andstructures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of theinvention. In addition, descriptions of well-known functions andconstructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of theinvention. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent invention is provided for illustration purpose only and not forthe purpose of limiting the invention as defined by the appended claimsand their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

For an Enhanced-Physical Downlink Control Chanel (E-PDCCH), in eachPhysical Resource Block (PRB) pair, the number of Resource Elements(REs) which actually may be used for transmitting the E-PDCCH isvariable. Accordingly, after Enhanced-Control Chanel Elements (E-CCEs)are divided in the PRB pair, the number of REs in an E-CCE is alsovariable. Moreover, in different subframes, the numbers of REs which maybe used for transmitting the E-PDCCH may be different. Accordingly, inthe different subframes, the numbers of REs contained in the E-CCEs aredifferent. For example, certain subframes are normal subframes, whileother subframes are Multicast Broadcast Single Frequency Network (MBSFN)subframes, or Channel State Information Reference Signal (CSI-RS) istransmitted in some of the subframes.

When the number of REs of the E-CCE is very small, it may result thatthe E-PDCCH of a certain aggregate level is unusable. For example, theE-PDCCH containing one Enhanced-Control Channel (E-CCH) may not be usedfor transmitting downlink control information of a certain DownlinkControl Information (DCI) format. In example 1 of FIG. 3, each E-CCHonly contains 21 REs on average, while the number of bits of the DCIformat may reach 50. Obviously, in a condition that the E-PDCCH usesQuad-Phase Shift Keying (QPSK) modulation, if the aggregate level is 1,it results that the coding rate of the DCI format is more than 1, maynot be transmitted successfully in this E-CCH.

Since the UE needs to detect multiple DCI formats at the same time, andthe numbers of bits of different DCI formats may be different, even thenumbers of REs in the E-CCEs are the same, it is possible that in oneE-CCH, certain DCI formats may be transmitted, but other DCI formats maynot be transmitted. Since the number of usable REs of differentsubframes may be different, it is possible that one DCI format which issupported to be transmitted in the E-PDCCH of a certain aggregate levelin some of the subframes may not be transmitted in the other subframes.

In a current configuration of a PDCCH search space, parameters of thePDCCH candidates (e.g., the number of PDCCH candidates, locations, etc.)are defined for each aggregate level respectively. That is, in eachsubframe, PDCCH blind detection for each DCI format is performed in thesame PDCCH search space. If the method for configuring the search spaceis still used for the E-PDCCH, since the number of REs in some of thesubframes is small, some of the aggregate levels are actually not usablefor some of the DCI formats and the number of the effective E-PDCCHcandidates is decreased. Thus, the flexibility with which the basestation transmits the E-PDCCH is reduced and the possibility that theE-PDCCHs of different User Equipments (UEs) block each other isincreased.

Based on the above analysis, exemplary embodiments of the presentinvention provide a method for configuring a search space of a downlinkcontrol channel.

FIG. 4 is a flowchart illustrating a method for configuring a searchspace for a downlink control channel according to an exemplaryembodiment of the present invention.

Referring to FIG. 4, when the parameters for E-PDCCH candidates of anE-PDCCH search space are configured, the parameters for E-PDCCHcandidates of each aggregate level are determined by the number of REsof the E-PDCCH in various subframes and/or the number of bits of the DCIformats in step 401.

When transmitting downlink control information, the base stationdetermines the parameters for E-PDCCH candidates according to the useddownlink frame and the DCI format. The base station chooses one E-PDCCHcandidate in the E-PDCCH search space corresponding to the parametersfor E-PDCCH candidates and transmits the downlink control information instep 402.

In step 403, the UE determines the parameters for E-PDCCH candidatesaccording to the current downlink subframe and the detected DCI formatand blindly detects the E-PDCCH candidates in the E-PDCCH search spacecorresponding to the parameters for E-PDCCH candidates.

The present invention provides two exemplary methods for implementingstep 401.

In a first exemplary method, when the E-PDCCH search space isconfigured, two sets of parameters for E-PDCCH candidates correspondingto various aggregation levels are configured and used for differentsubframes and/or different DCI formats. This first exemplary method maybe implemented by one of the following three exemplary sub-methods.

In a first exemplary sub-method, different E-PDCCH parameters areconfigured for different subframes according to the numbers of REsavailable for the E-PDCCH in various subframes. All of the DCI formatsin the same subframe use the same parameters for E-PDCCH candidates.

The first exemplary sub-method could be applied to a condition in which,in some of the subframes, the number of REs available for the E-PDCCH issufficient, thus, the E-PDCCH for each aggregate level may carry eachDCI format. In the other kind of subframes, the number of REs which maybe used for the E-PDCCH is not sufficient, thus, the E-PDCCH of acertain aggregate level (e.g., the aggregate level is 1) may not carryany one of the DCI formats. The two sets of the parameters for E-PDCCHcandidates may be respectively configured for the two kinds ofsubframes.

For example, if it is assumed that each PRB pair is divided into 4E-CCEs, and a localized E-PDCCH is mapped in the same PRB pair (i.e.,according to a tree structure), the aggregate levels of the E-PDCCH maybe L_(E)ε{1,2,4}. A system may configure the two sets of the numbers ofthe E-PDCCH candidates, denoted as M_(E) ⁽¹⁾ε{M_(E,1) ⁽¹⁾M M_(E,2) ⁽¹⁾,M_(E,4) ⁽¹⁾} and M_(E) ⁽²⁾ε{M_(E,1) ⁽²⁾M M_(E,2) ⁽²⁾, M_(E,4) ⁽²⁾}.Further, it may be assumed that the aggregate levels includeL_(E)ε{1,2,4}, the number of E-PDCCH candidates is taken as an example,one set of the numbers of the E-PDCCH candidates may be set as M_(E)⁽¹⁾ε{4,4,2} and used for the subframe in which the E-PDCCH of eachaggregate level may carry each DCI format. The other set of numbers ofthe E-PDCCH candidates may be set as M_(E) ⁽²⁾ε{0,6,4} and used for thesubframe in which the E-PDCCH, having an aggregate level of 1, may notcarry any one of the DCI formats. In the above example, when the otherset of numbers of the E-PDCCH candidate are set, the unusable E-PDCCHcandidates of the aggregative level 1 are respectively assigned to theaggregate levels 2 and 4 to guarantee that the total number of theE-PDCCH candidates is not changed. Alternatively, other ways may beused, as long as the total number of the E-PDCCH candidates is notchanged or is changed little. For example, according to a rightwardshift, the other set of numbers of the E-PDCCH candidates may be set asM_(E) ⁽²⁾ε{0,4,4}.

It can be seen that, according to the prior art, there is only one setof parameters for E-PDCCH candidates. For the subframe in which thenumber of REs is not enough, the E-PDCCH candidate having an aggregatelevel of 1 is unusable. Thus, the total number of the actual usableE-PDCCH candidates is decreased. However, in an exemplary implementationof the present invention, two sets of parameters for E-PDCCH candidatesare configured. As an example, the E-PDCCH candidates, which areunusable in a certain aggregate level, are assigned to other aggregatelevels to guarantee that the total number of E-PDCCH candidates is notchanged or changed little. Thus, the flexibility with which the basestation transmits the E-PDCCH is increased and the possibility that theE-PDCCHs of different UEs block each other is reduced.

In the exemplary sub-method, when the base station transmits thedownlink information and when the UE receives the downlink controlinformation, the set of the parameters for E-PDCCH candidates to be usedis determined according to the numbers of REs available for the E-PDCCHin various subframes.

In a second exemplary sub-method, different parameters for E-PDCCHcandidates are configured for different DCI formats according to thenumber of bits of each DCI format. The same DCI format uses the sameparameters for E-PDCCH candidates in all of the subframes.

For example, it is assumed that the UE is configured with three DCIformats of which the number of bits is different with each other, whichinclude the DCI format of downlink Multiple Input Multiple Output(MIMO), the DCI format of uplink MIMO and a fallback DCI format. Theparameters for E-PDCCH candidates may be respectively defined for theabove three DCI formats. Alternatively, in order to simplify operation,the DCI formats may be grouped. Different groups use differentparameters for E-PDCCH candidates. For example, the DCI format ofdownlink MIMO and the DCI format of uplink MIMO use the same parametersfor E-PDCCH candidates, while a downlink fallback DCI format and the DCIformat of an uplink single antenna use the same parameters for E-PDCCHcandidates.

The exemplary sub-method could be applied to the condition in which thenumber of REs that may be used for the E-PDCCH in all of the subframesare equal or close, according to the method for dividing the E-CCEs inthe subframe. In one case, the E-PDCCH of a certain aggregate level(e.g., the aggregate level is 1) may carry one part of the DCI formats(e.g., the fallback DCI format), but in the other case, the E-PDCCH ofthe certain aggregate level may not carry the other DCI formats (e.g.,all of the DCI formats except the fallback DCI format). Two sets of theparameters for E-PDCCH candidates may be respectively configured for thetwo cases. For example, the aggregate levels include L_(E)ε{1,2,4}. Thenumber of the E-PDCCH candidates is taken as an example. One set of thenumbers of the E-PDCCH candidates may be set as M_(E) ⁽¹⁾ε{4,4,2}, usedfor the DCI format that may be carried by the E-PDCCH having anaggregate level of 1. The other set of the E-PDCCH candidate may be setas M_(E) ⁽²⁾ε{0,6,4}, used for the DCI format which may not be carriedby the E-PDCCH having an aggregate level of 1. In the above example,when the other set of numbers of the E-PDCCH candidate is set, theunusable E-PDCCH candidates of the aggregative level 1 are assignedrespectively to the aggregate levels 2 and 4 to guarantee that the totalnumber of the E-PDCCH candidates is not changed. Alternatively, otherways may be used, as long as the total number of the E-PDCCH candidatesis not changed or is changed little. For example, according to arightward shift, the other set of the numbers of the E-PDCCH candidatesare set as M_(E) ⁽²⁾ε{0,4,4}.

In exemplary embodiments of the present invention, when the two sets ofparameters for E-PDCCH candidates are set, the total number of E-PDCCHcandidates is not changed or changed little (e.g., the unusable E-PDCCHcandidates for a certain aggregate level are assigned to the otheraggregate levels). Thus, the flexibility with which the base stationtransmits the E-PDCCH is increased and the possibility that the E-PDCCHsof different UEs block each other is reduced.

In a third exemplary sub-method, different parameters for E-PDCCHcandidates are configured for different DCI formats in differentsubframes according to the number of REs which may be used for theE-PDCCH in each subframe and the number of bits of the DCI format.

For example, at least two sets of numbers of the E-PDCCH candidates fora different search space are configured, according to whether theE-PDCCH of a certain aggregate level may carry at least one DCI formatto be transmitted. Thus, the set of the parameters for E-PDCCHcandidates to be used may be obtained implicitly, according to thenumber of REs which may be used for the E-PDCCH in a subframe and thenumber of bits of the DCI format. Alternatively, if the DCI formats aregrouped, the set of parameters for E-PDCCH candidates may be obtainedimplicitly for one subframe and one group of the DCI formats.

For example, an aggregate level of 1 is taken as a condition toconfigure at least two sets of numbers of E-PDCCH candidates.

When the E-PDCCH having an aggregate level of 1 may carry at least oneDCI format, one set of numbers of E-PDCCH candidates is configured,e.g., the numbers of the E-PDCCH candidates of various aggregate levelsin the search space are respectively configured as M_(E) ⁽¹⁾ε{4,4,2}.The set of numbers of E-PDCCH candidates may apply to two cases. In afirst case, the number of REs which may be used for the E-PDCCH islarge, so that all of the possible DCI formats may be carried in oneE-CCH. In a second case, one E-CCE in the subframe may carry some of theDCI formats, e.g., the fallback DCI format. If at least one DCI formatmay not be carried by the E-PDCCH having an aggregate level of 1, theother set of numbers of E-PDCCH candidates are configured, e.g., thenumbers of the E-PDCCH candidates of various aggregate levels in thesearch space are respectively M_(E) ⁽¹⁾ε{4,4,2}. The set of numbers ofE-PDCCH candidates may also apply to two cases. In a first case, thenumber of REs which may be used for the E-PDCCH is small, thus, any DCIformat may not be carried in one E-CCH. In a second case, one E-CCH inthe subframe may not carry a part of the DCI formats, e.g., the DCIformat of the downlink and uplink MIMO. In the above example, when theother set of numbers of E-PDCCH candidates are configured, the unusableE-PDCCH candidates of the aggregative level 1 are assigned respectivelyto the aggregate levels 2 and 4 to guarantee that the total number ofthe E-PDCCH candidates is not changed. Alternatively, other methods maybe used, as long as the total number of E-PDCCH candidates is notchanged or is changed little. For example, according to a rightwardpanning, the other set of numbers of E-PDCCH candidates are set as M_(E)⁽²⁾ε{0,4,4}.

In exemplary embodiments of the present invention, when the two sets ofthe parameters for E-PDCCH candidates are set, the total number ofE-PDCCH candidates is not changed or changed little (e.g., the unusableE-PDCCH candidates for a certain aggregate level are assigned to theother aggregate levels). Thus, the flexibility with which the basestation transmits the E-PDCCH is increased and the possibility that theE-PDCCHs of different UEs block each other is reduced.

In a second exemplary method, a workable aggregate level is introduced.

The working aggregate level means that, for the number of REs for acertain E-CCE, if the E-PDCCH of one aggregate level may carry a certainDCI format, the aggregate level is the workable aggregate level for theDCI format. Herein, the condition in which the E-PDCCH of the aggregatelevel may carry the DCI format may be that in the aggregate level, aneffective coding rate of the DCI format is less than a certain thresholdvalue, e.g., 0.8 or 1. The threshold value may be pre-defined, or betransmitted by a broadcast signal or an RRC message.

Alternatively, the workable aggregate level may be defined for thesubframe, i.e., the same definition is used for various DCI formats inone subframe. For example, according to the E-CCE definition, if thenumber of REs of the E-CCE in the subframe is large and one E-CCE maycarry the E-PDCCH, the workable aggregate level which is uniformlydefined for the subframe includes various aggregate levels started fromthe aggregate level which is 1, e.g., the aggregate level which is 1, 2,4 or 8. If the number of REs of the E-CCE in the subframe is small, atleast two E-CCEs are required to carry the E-PDCCH, the workableaggregate levels which are uniformly defined for the subframe arevarious aggregate levels started from the aggregate level which is 2,e.g., the aggregate level which is 2, 4, 8 or 16. Alternatively, if thenumber of REs of the E-CCE in the subframe is very small and two E-CCEsmay not carry the E-PDCCH, it may be considered that the workableaggregate levels which are uniformly defined for the subframe includevarious levels started from the aggregate level which is 4 or higher,e.g., the aggregate level which is 4, 8, 16 or 32.

After the concept of the workable aggregate level is introduced, only aset of standard parameters for E-PDCCH candidates may be defined. Bytranslating the set of the standard parameters for E-PDCCH candidates,the parameters for E-PDCCH candidates of the workable aggregate levelare obtained. Determination of the numbers of E-PDCCH candidates ofvarious aggregate levels is taken as an example. In an exemplarytranslation method, the number of E-PDCCH candidates of the aggregatelevel less than the minimal workable aggregate level is set as 0, ordirectly set as unusable, the number of E-PDCCH candidates of theminimal workable aggregate level is set as the first item of the numberof the standard E-PDCCH candidates, the number of E-PDCCH candidates ofthe second minimal workable aggregate level is set as the second item ofthe number of the standard E-PDCCH candidates, and so on. The aboveexemplary translation method may apply to one DCI format, i.e.,different DCI formats may have different workable aggregate levels,thus, may be configured with different parameters for E-PDCCHcandidates. Also, the above exemplary translation method may apply toall of the DCI formats in one subframe, i.e., there are differentworkable aggregate levels in different subframes, thus, the differentparameters for E-PDCCH candidates are configured to different subframes.

For example, if the number of E-PDCCH candidates in the parameters forE-PDCCH candidates is considered, a set of numbers of the standardE-PDCCH candidates is defined as M_(E)ε{4,4,2}.

An example of the method in which the workable aggregate level isdefined for each DCI format is described as follows. If the E-PDCCHhaving an aggregate level of 1 may carry the DCI format, the minimalworkable aggregate level is 1. Accordingly, the aggregate levels areL_(E)ε{1,2,4}, and the number of E-PDCCH candidates corresponding to theaggregate levels respectively are M_(E)ε{4,4,2}.

If the E-PDCCH having an aggregate level of 1 may not carry the DCIformat, and the E-PDCCH having an aggregate level of 2 may carry the DCIformat, the aggregate level of 2 is the minimal workable aggregatelevel. In that case, the aggregate levels are L_(E)ε{1,2,4}. The abovenumbers of standard E-PDCCH candidates are shifted rightward and thenumber of E-PDCCH candidates respectively corresponding to the aggregatelevels are obtained respectively as M_(E)ε{0,4,4}. In addition, if thenumber of REs of the E-CCE is small, the E-PDCCH having an aggregatelevel of 8 is allowed to be used. Thus, the aggregate levels areL_(E)ε{1,2,4,8}, and the aggregate level of 2 is the minimal workableaggregate level. The number of E-PDCCH candidates is shifted rightward,and the number of E-PDCCH candidates respectively corresponding to theaggregate levels are obtained respectively as M_(E)ε{0,4,4,2}.

An example of the method in which the workable aggregate level isdefined for the subframe is described as follows. If the number of REsof the E-CCE in the subframe is large, one E-CCE may carry the E-PDCCH,and the minimal workable aggregate level is 1 for the subframe. Thus,the aggregate levels are L_(E)ε{1,2,4}, and the number of E-PDCCHcandidates corresponding to the aggregate levels respectively isM_(E)ε{4,4,2}.

If the number of REs of the E-CCE in the subframe is small, at least twoE-CCEs may carry the E-PDCCH, and the minimal workable aggregate levelis 2. Thus, the aggregate levels are L_(E) ε{1,2,4} the number of theabove standard E-PDCCH candidates is shifted rightward, and the numberof E-PDCCH candidates respectively corresponding to the aggregate levelsis obtained respectively as M _(E)ε{0,4,4}. In addition, if the numberof REs of the E-CCE is small, the E-PDCCH having an aggregate level of 8is allowed to be used, i.e., the aggregate levels are L_(E)ε{1,2,4,8},the minimal workable aggregate level is 2, the number of alternativeaggregate levels may be shifted rightward, and the number of E-PDCCHcandidates respectively corresponding to the aggregate levels isobtained respectively as M_(E)ε{0,4,4,2}.

In the second exemplary method, only one set of parameters for E-PDCCHcandidates is configured, yet the same effect as to configure multiplesets of the E-PDCCH parameters is reached as in the first exemplarymethod.

The exemplary parameters for E-PDCCH candidates described in the presentinvention may be configured by the RRC signal, thus, each UE may beconfigured respectively. Alternatively, the exemplary parameters forE-PDCCH candidates may be configured by the broadcast signal, thus, allof UEs in a cell are applicable. Alternatively, the parameters forE-PDCCH candidates may be pre-defined, e.g., in a specification.

Referring again to step 401, the localized E-PDCCH and the distributedE-PDCCH may be handled independently, i.e., variation of the number ofREs of the PRB pair only impacts the number of E-PDCCH candidates in thesearch space of the localized E-PDCCH, and does not impact the number ofE-PDCCH candidates in the search space of the distributed E-PDCCH.Alternatively, the variation of the number of REs of the PRB pair mayimpact both the localized E-PDCCH and the distributed E-PDCCH. Forexample, the variation of the number of REs of the PRB pair causes thevariation of the number of E-PDCCH candidates in the search space of thelocalized E-PDCCH. Accordingly, the number of E-PDCCH candidates in thesearch space of the distributed E-PDCCH is varied. Thus, the totalnumber of E-PDCCH candidates in the UE search space is not varied or isvaried little.

For example, when the localized E-PDCCH having an aggregate level of 1may carry at least one DCI format, one set of numbers of the localizedE-PDCCH candidates and one set of numbers of the distributed E-PDCCHcandidates are configured. For example, the number of various localizedE-PDCCH candidates in the search space is configured as M_(E,L)⁽¹⁾ε{4,4,1}, and the number of various distributed E-PDCCH candidates inthe search space is configured as M_(E) ⁽¹⁾ε{2,2,1}.

If at least one DCI format may not be carried by the localized E-PDCCHhaving an aggregate level of 1, the other set of numbers of thelocalized E-PDCCH candidates and the other set of numbers of thedistributed E-PDCCH candidates are configured. For example, the numberof various localized E-PDCCH candidates in the search space isconfigured as M_(E,L) ⁽¹⁾ε{0,4,1}, the number of various distributedE-PDCCH candidates in the search space is configured as M_(E,D)⁽¹⁾ε{6,2,1}.

In the above exemplary method in which two sets of E-PDCCH candidateparameters are configured, the number of the localized E-PDCCHcandidates which may not work is assigned to the distributed E-PDCCHcandidate corresponding to the same aggregate level. Alternatively, onepart of the number of the localized E-PDCCH candidates which may notwork is assigned to the other localized E-PDCCH candidates, the otherpart of the number of the localized E-PDCCH candidates which may notwork is assigned to the distributed E-PDCCH candidates corresponding tothe same and/or the different aggregate level.

For example, when the other set of the localized E-PDCCH candidates andthe other set of the distributed E-PDCCH candidates are configured, thenumber of the localized E-PDCCH candidates of various aggregate levelsin the search space is M_(E,L) ⁽¹⁾ε{0,6,1}, and the number of thedistributed E-PDCCH candidates of various aggregate levels in the searchspace is M_(E,D) ⁽¹⁾ε{4,2,1}. In the above example, it is assumed thatthe total number of the localized E-PDCCH candidates and distributedE-PDCCH candidates in the two sets of the parameters are equal. Ofcourse, it is to be understood that the present invention is not solimited in that the total number of the localized E-PDCCH candidates anddistributed E-PDCCH candidates in the two sets of the parameters neednot be equal.

Exemplary embodiments of the present invention also provide a userequipment and a base station.

FIG. 5 is a schematic diagram illustrating a structure of a userequipment according to an exemplary embodiment of the present invention.

Referring to FIG. 5, the user equipment includes a parameter selectionmodule 510 and a blind detection module 520.

The parameter selection module 510 is adapted to determine theparameters for E-PDCCH candidates according to a current downlinksubframe and a detected DCI format.

The blind detection module 520 is adapted to blindly detect an E-PDCCHin an E-PDCCH search space corresponding to the parameters for E-PDCCHcandidates determined by the parameter selection module.

FIG. 6 is a schematic diagram illustrating a structure of a base stationaccording to an exemplary embodiment of the present invention.

Referring to FIG. 6, the base station includes a parameter selectionmodule 610 and a sending module 620.

The parameter selection module 610 is adapted to determine theparameters for E-PDCCH candidates according to a current downlinksubframe and a detected DCI format.

The sending module 620 is adapted to select an E-PDCCH candidate in anE-PDCCH search space corresponding to the parameters for E-PDCCHcandidates selected by the parameter selection module, and to senddownlink control information.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the of theinvention as defined by the appended claims and their equivalents.

What is claimed is:
 1. A method for configuring a search space of adownlink control channel, the method comprising: determining parametersfor Enhanced Physical Downlink Control Channel (E-PDCCH) candidates ofeach of aggregate levels according to the number of Resource Elements(RE) in a subframe and/or the number of bits of Downlink ControlInformation (DCI) formats when configuring the parameters for E-PDCCHcandidates of an E-PDCCH search space; determining, by a User Equipment(UE), the parameters for E-PDCCH candidates according to a currentdownlink subframe and a detected DCI format; and detecting blindly, bythe UE, the E-PDCCH candidates in the E-PDCCH search space correspondingto the parameters for E-PDCCH candidates.
 2. The method according toclaim 1, wherein, at least two sets of parameters for E-PDCCH candidatesare configured respectively for the aggregate levels.
 3. The methodaccording to claim 2, wherein, different parameters for E-PDCCHcandidates are configured for different subframes according to thenumber of REs available for the E-PDCCH in each of the subframes, andthe same parameters for E-PDCCH candidates are adopted for all of theDCI formats in the same subframe.
 4. The method according to claim 2,wherein, different parameters for E-PDCCH candidates are configured fordifferent DCI formats according to the number of bits of each DCIformat, and the same parameters for E-PDCCH candidates are used for oneDCI format in all of the subframes.
 5. The method according to claim 2,wherein, different parameters for E-PDCCH candidates are configured fordifferent DCI formats in different subframes according to the number ofREs which may be used for the E-PDCCH in each subframe and the number ofbits of each DCI format.
 6. The method according to claim 4, furthercomprising: grouping the DCI formats; and configuring the sameparameters for E-PDCCH candidates for each group of the DCI formats. 7.The method according to claim 2, wherein, at least two sets of theparameters for E-PDCCH candidates corresponding to each aggregate levelare configured according to whether the E-PDCCH of an aggregate levelmay carry at least one DCI format to be transmitted.
 8. The methodaccording to claim 1, further comprising: configuring a single set ofstandard parameters for E-PDCCH candidates; determining workableaggregate levels according to the number of the REs which may be usedfor the E-PDCCH in the subframe; and obtaining the parameters forE-PDCCH candidates of each aggregate level by translating the standardparameters for E-PDCCH candidates.
 9. The method according to claim 8,wherein, the workable aggregate level is determined for each DCI format,and the parameters for E-PDCCH candidates of each aggregate level areobtained, or the workable aggregate level is determined uniformly forall DCI formats in each subframe, and the parameters for E-PDCCHcandidates of each aggregate level are obtained.
 10. The methodaccording to claim 8, wherein the translating of the standard parametersfor E-PDCCH candidates comprises: setting the number of the E-PDCCHcandidates of the aggregate level less than the minimal workableaggregate level to 0; setting the number of the E-PDCCH candidates ofthe minimal workable aggregate level as the first item of the number ofthe standard E-PDCCH candidates; and setting the number of the E-PDCCHcandidates of the second minimal workable aggregate level as the seconditem of the number of the standard E-PDCCH candidates.
 11. The methodaccording to claim 2, wherein, a variation of the number of the REsavailable for the E-PDCCH in a PRB pair influences only the number ofthe E-PDCCH candidates in a localized E-PDCCH search space, or avariation of the number of the REs available for the E-PDCCH in a PRBpair influences the number of the E-PDCCH candidates both in a localizedE-PDCCH search space and that in a distributed E-PDCCH search space. 12.A User Equipment (UE) comprising: a parameter selection moduleconfigured to determine parameters for E-PDCCH candidates according to acurrent downlink subframe and a detected DCI format; and a blinddetection module adapted to blindly detect an E-PDCCH in an E-PDCCHsearch space corresponding to the parameters for E-PDCCH candidatesdetermined by the parameter selection module.
 13. The UE according toclaim 12, wherein, at least two sets of parameters for E-PDCCHcandidates are configured respectively for the aggregate levels.
 14. TheUE according to claim 13, wherein, different parameters for E-PDCCHcandidates are configured for different subframes according to thenumber of REs available for the E-PDCCH in each of the subframes, andthe same parameters for E-PDCCH candidates are adopted for all of theDCI formats in the same subframe.
 15. The UE according to claim 13,wherein, different parameters for E-PDCCH candidates are configured fordifferent DCI formats according to the number of bits of each DCIformat, and the same parameters for E-PDCCH candidates are used for oneDCI format in all of the subframes.
 16. The UE according to claim 13,wherein, different parameters for E-PDCCH candidates are configured fordifferent DCI formats in different subframes according to the number ofREs which may be used for the E-PDCCH in each subframe and the number ofbits of each DCI format.
 17. A Base Station (BS) comprising: a parameterselection module adapted to determine parameters for E-PDCCH candidatesaccording to a current downlink subframe and a detected DCI format; anda sending module adapted to select an E-PDCCH candidate in E-PDCCHsearch space corresponding to the parameters for E-PDCCH candidatesselected by the parameter selection module, and to send downlink controlinformation.
 18. The BS according to claim 17, wherein, at least twosets of parameters for E-PDCCH candidates are configured respectivelyfor the aggregate levels.
 19. The BS according to claim 18, wherein,different parameters for E-PDCCH candidates are configured for differentsubframes according to the number of REs available for the E-PDCCH ineach of the subframes, and the same parameters for E-PDCCH candidatesare adopted for all of the DCI formats in the same subframe.
 20. The BSaccording to claim 18, wherein, different parameters for E-PDCCHcandidates are configured for different DCI formats according to thenumber of bits of each DCI format, and the same parameters for E-PDCCHcandidates are used for one DCI format in all of the subframes.
 21. TheBS according to claim 18, wherein, different parameters for E-PDCCHcandidates are configured for different DCI formats in differentsubframes according to the number of REs which may be used for theE-PDCCH in each subframe and the number of bits of each DCI format.